The invention relates generally to the field of magnetic bubble technology (MBT) and more particularly to arrangements for improving the quality of readout in data processing systems utilizing the capabilities of single-walled magnetic domain devices.
The continuing evolution of MBT has now reached the point where large-scale application to various data processing tasks is practicable. Current interest in MBT is due primarily to the prospect of extremely high bit-packing density, low power consumption and reliability for low-cost mass memories where the speed of access is secondary to capacity or nonvolatility at a reasonable size and cost per bit.
Briefly, MBT involves the creation, propagation and logical manipulation of single-walled magnetic domains in specially prepared magnetic materials. The application of a static uniform magnetic bias field orthagonal to a sheet of magnetic material having suitable uniaxial anisotropy causes the normally random pattern of magnetic domains to shrink into short cylindrical configurations called bubbles whose common polarity is opposite that of the bias field. The bubbles repel each other and can be moved or propagated by a magnetic field in the plane of the sheet. The term bubble used herein is intended to encompass any single-walled magnetic domain, i.e., any domain having a boundary which closes on itself.
Many schemes now exist for propagating bubbles along predetermined channels. One propagation system includes permalloy circuit elements shaped like military service stripes or "chevrons" spaced end-to-end in a thin layer over a sheet of magnetic material. The magnetic drive field is continuously rotating in the plane of the sheet. This propagation system is termed "field-accessed" as distinguished from other systems employing loops of electrical conductors disposed on the sheet. The operation of both types of propagation systems is described in the existing literature on MBT.
The use of MBT in data processing stems from the fact that magnetic bubbles can be propagated through channels at a precisely determined rate enabling the creation of uniform streams of bubbles in which the presence or absence of a bubble indicates a binary "1" or "0" at a corresponding bit position within the stream. The use of MBT for performing logic operations is based on the fact that closely adjacent magnetic bubbles tend to repel each other. Thus, if alternate paths with varying degrees of preference are built into a bubble circuit, the direction which a bubble on one channel ultimately takes can be influenced by the presence or absence of a bubble on another nearby channel. Several types of logic circuits are discussed in the copending application Ser. No. 283,267, R. C. Minnick et al, filed Aug. 24, 1972, entitled "Magnetic Bubble Logic Family," assigned to the assignee of the present application. A substantial portion of the disclosure in this copending application has been published in the Proceedings of the Sept. 19, 1972, Wescon Conference in a paper entitled "Magnetic Bubble Logic" by R. C. Minnick et al. These disclosures are incorporated in full by reference into this application.
One of the most difficult problems facing practical implementation of MBT is readout. Despite efforts to minimize readout by incorporating logic in the memory to enhance the informational content of each bit, a reliable readout is still, of course, necessary at some point to utilize the information carried by the bubble bits. In the past, optical devices utilizing the Faraday or Kerr effect and magnetoresistive or magnetoconductive devices, for example, employing the Hall effect, have been used as well as simple conductor loops. The basic problem, however, is that the small magnetic field associated with a single miniscule magnetic bubble propagating along a channel is barely distinguishable from the background noise which, by itself, represents the absence of a bubble; and therefore extremely sensitive detectors, entailing such problems as matching, reliability and cost, must be used to effect readout. While detection can be improved by expanding the size of an individual bubble with known devices referred to in the literature as "bubble-stretchers," the physical limitations of this technique have restricted its success.